1. Field of the Invention
The present invention relates to methods for fabricating integrated circuits and, more particularly, to an apparatus and method for fabricating a chirped grating in a surface emitting distributed feedback semiconductor laser diode device.
2. Description of the Prior Art
Since the early 1970's, surface emitting laser diode devices with a variety of different waveguide grating structures have been explored. Of those explored, the distributed Bragg reflector (DBR) and the distributed feedback (DFB) devices have attracted the most interest. DBR laser devices have gratings at the ends of a pumped region to provide a feedback of photon radiation back into the pumped region. DFB laser devices have a continuous grating along one side of a pumped region to provide a feedback of photon radiation back into the pumped region. To date, mainly DBR devices have been demonstrated. However, due to a high thermal resistance from a substrate side heat sink bonding configuration, these devices have had to be operated in a pulsed mode. More recently, DFB devices have been demonstrated. These devices offer the promise of reliable high-power continuous wave operation due to an epitaxial side heat sink bonding configuration design.
To date, surface emitting distributed feedback semiconductor laser devices have been fabricated with a second order, constant periodicity grating etched into a cladding layer surface of a semiconductor wafer. This type of grating can be fabricated by creating an optical standing wave on a photoresist coated wafer surface using two beam interference. The wafer is then treated with an ion milling process and a chemical etching process to transfer the photoresist exposed grating pattern into the cladding layer surface of the semiconductor wafer.
Devices with second order, constant periodicity gratings allow photon radiation, originating in an electrically pumped region, to be deflected from the grating surface back into the pumped region through second order diffraction. This second order diffracted photon radiation is deflected in two opposite directions, each direction being normal from one side of a grating groove. Due to the constant periodicity of the grating, the amplitudes of these oppositely directed photon fields are antisymmetric to one another about a center of the grating surface. The opposition field direction allows for their destructive interference. The antisymmetry of the amplitudes of these fields produces a total destructive interference at the center of the grating surface and a corresponding antisymmetric superposition of these fields about this grating surface center. This antisymmetric superposition of oppositely directed, second order diffracted photon fields results in a device output beam that exhibits an antisymmetric, longitudinal mode near-field output intensity profile and a corresponding symmetric, double-lobed longitudinal mode far-field output intensity profile, all symmetries being about the center of the grating surface.
Surface emitting distributed feedback semiconductor laser devices provide significant advantages over DBR devices because of their ability to operate in a continuous wave mode at high power. However, destructive interference of second order diffracted photon radiation reduces the power efficiency of these DFB devices. Also, the output beam intensity is split between two far-field lobes. It is therefore desirable to increase the power efficiency of these devices while concentrating the output beam intensity into a single far-field lobe.